1. Field of the Invention
The present invention is directed to a method for prescribing a temporally variable referencequantity for a gradient field of a gradient coil of a magnetic resonance tomography apparatus.
2. Description of the Prior Art
Magnetic resonance tomography is a known technology for acquiring images of the body interior of a living examination subject. To that end, rapidly switched gradient fields that are generated by a gradient coil system are superimposed on a static basic magnetic field in a magnetic resonance tomography apparatus. The magnetic resonance tomography apparatus also has a radio-frequency system that emits radio-frequency signals into the examination subject for triggering magnetic resonance signals. The radio-frequency system also registers the generated magnetic resonance signals from which magnetic resonance images are produced.
The gradient coil system usually is surrounded by conductive structures wherein eddy currents are induced by the switched gradient fields. Examples of such conductive structures are the inner cold shield of a super-conductive basic field magnet system, copper foil of a radio-frequency shielding, and the gradient coil system itself. The fields generated by the eddy currents are unwanted because, without counter-measures, measures, they weaken the gradient field and distort its time curve. This leads to degradations in the quality of magnetic resonance images.
The aforementioned eddy current fields can be compensated to a certain extent by an appropriate pre-distortion of a referencecurrent quantity of the gradient coil. Only eddy current fields that similarly track the gradient field in the mathematical sense, i.e. that are similar to the gradient field in terms of their field course, can be compensated by the pre-distortion. Since, however, eddy currents also exist that do not similarly track the gradient field, additional spatial field distortions referred to as higher order distortions arise. Actively shielded gradient coils are utilized in order to largely compensate these latter field distortions.
The basic functioning of known pre-distortion techniques described, for example, in U. S. Pat. No. 4,585,995 and U.S. Pat. No. 4,703,275. The calculation of the pre-distortion is essentially based on the perception that excited and decaying eddy currents can be described by a specific number of e-functions with different time constants. Transferred to an electrical network for the compensation of eddy currents, this means that the pre-distortion can be implemented with filters having different limit frequencies. The setting of the time constants or limit frequencies ensues, for example, by an operator who determines the optimum values at the installed magnetic resonance tomography apparatus by step-by-step modification of settings of the pre-distortion, with repeated review and re-adjustment. Depending on the experience of the operator, this setting is more or less time-consuming. In other known systems, the setting of the time constants or limit frequencies ensues automatically, as disclosed, for example, in U.S. Pat. No. 4,928,063.
In the execution of a measuring sequence, the pre-distortion of the referencecurrent quantity is implemented continuously during the entire time of the measuring sequence. For example, the pre-distortion is thereby implemented in a digital signal processor of a gradient system according to supporting points before a digital-to-analog conversion of the referencecurrent quantity. This represents a considerable calculating out lay for the digital signal processor and accordingly requires a powerful, and thus expensive, processor. Moreover, extra power capacity ([power reserves]) must be kept available due to the pre-distortion, which necessitates a more powerful and thus more expensive dimensioning of the gradient amplifiers.
It is an object of the invention to provide a method for prescribing a temporally variable referencequantity for a gradient field of a gradient coil of a magnetic resonance tomography apparatus that reduces the aforementioned disadvantages and, in particular, reduces the necessity of maintaining power reserves in the gradient amplifiers and simplifies the calculation of the pre-distortion.
The above object is achieved in accordance with the principles of the present invention in a method for prescribing a temporally variable reference quantity for a gradient field produced by a gradient coil in a magnetic resonance tomography apparatus, wherein the reference quantity exhibits no pre-distortion for compensating eddy currents in a first time segment that is not a transmission or reception time segment of the radio-frequency signal in the tomography apparatus, and wherein the reference quantity exhibits a pre-distortion that compensates eddy currents in a second time segment that is a transmission or reception time segment for the radio-frequency signal.
Because the referencequantity does not exhibit any pre-distortion compensating eddy currents in a first time segment that is not a transmission or reception time segment of the radio-frequency signal, and because the reference quantity exhibits a pre-distortion compensating eddy currents in a second time segment that is a transmission or reception time segment of the radio-frequency signal, a gradient amplifier supplying the gradient coil is freed from maintaining power reserves during the first time segment. Foregoing a pre-distortion during the first time segment is particularly possible because an exact adherence of the gradient field to a prescribable curve shape is not important for the generation of magnetic resonance images in time segments wherein no radio-frequency signal is transmitted or received; rather, the exact adherence to a prescribable gradient time area or, mathematically expressed, the adherence to a prescribable, 0th time moment of the gradient field, is important. Further, a supporting location-dependent calculation of the pre-distortion for first time segments is completely eliminated during the course of a measuring sequence implementation. The calculating outlay to be performed is thereby reduced.
A great number of measuring sequences are characterized by the gradient field having a temporally constant gradient during the second time segment wherein a radio-frequency signal is transmitted or received. Due to this fact, the pre-distortion is implemented during the second time segment as follows with a reduced calculating outlay. As a result of the gradient field having a temporally constant gradient, no new eddy current is excited during the second time segment, so that only decaying eddy currents need be compensated for the pre-distortion, these having been excited in chronologically preceding time segments.